As known in the art, automated working machines, such as machine tools and industrial robots, apply forces to workpieces and are themselves subjected to external forces because of the manner in which these machines operate. In this case, it is necessary for the working machines to detect external forces and moments applied to the machines and to perform control corresponding to the detected external forces and moments. In order to perform the control, corresponding to the detected external forces and moments, with a high degree of precision, it is required to accurately detect the external forces and moments.
In view of this situation, various types of force sensors have been proposed to date. Generally, the force sensors can be classified, according to the detection scheme employed, into elastic-type force sensors and equilibrium-type force sensors. The elastic-type force sensors measure a force on the basis of an amount of deformation proportional to the external force, while the equilibrium-type force sensors measure a force by balancing it with a known force.
Also known are force sensors whose structure is based on the principle that a plurality of strain resistance elements are provided in parts of a strain-generating body that is elastically deformable in response to an external force applied thereto. When an external force is applied to the strain-generating body of the force sensor, electrical signals corresponding to the degree of deformation (stress) of the strain-generating body are output from the plurality of strain resistance elements. Forces that have two or more components and are applied to the strain-generating body can be detected on the basis of these electrical signals, and a stress produced in the force sensor is calculated on the basis of the electrical signals.
Among examples of the conventionally-known elastic-type force sensors are six-axis force sensors, each of which includes a plurality of strain resistance elements provided in parts of a strain-generating body. The six-axis force sensors divide an external force applied thereto into stress components (i.e., forces Fx, Fy, Fz) in respective axial direction of three axes (i.e., X-axis, Y-axis and Z-axis) of an orthogonal coordinate system and into torque components (i.e., moments Mx, My, Mz) about the respective axes, and it detects the external force as six axis components.
The inventors of the present invention etc. proposed a six-axis force sensor, having a novel construction, in Japanese Patent Laid-Open Publication No. 2003-207405. This proposed six-axis force sensor can provide a solution to the problem of interference from other axes (i.e., inter-axis interference problem) that prevents individual components (i.e., forces and moments) of an external force, applied to the strain-generating body, from being accurately separated from one another or resolved with good precision. In the proposed six-axis force sensor, a plurality of strain resistance elements are integrally assembled in a predetermined arrangement or layout pattern in parts of a strain-generating body on a semiconductor substrate by using semiconductor manufacturing processing. The proposed six-axis force sensor is formed using the semiconductor substrate of a substantially square planar shape, which includes: a supporting part located in an outer peripheral portion of the semiconductor substrate, an operating part located in a central portion of the semiconductor substrate and having a substantially square shape, and connecting parts connecting the four side of the square operating part and corresponding portions of the supporting part. The strain resistance elements are provided on boundary areas between the individual sides of the square operating part and the connecting parts. The proposed six-axis force sensor is arranged to solve the “inter-axis interference” problem through an improvement in the configuration of parts of the strain-generating body and optimization of the layout pattern of the plurality of strain resistance elements.
However, the six-axis force sensor proposed or disclosed in the No. 2003-207405 publication would present the following problem. Namely, when a force Fz is applied in the Z-axis direction to the operating part of the semiconductor substrate of the disclosed six-axis force sensor, the operating part is displaced and deforms in the Z-axis direction. In this case, individual terms of a mathematical expression for calculating a sensor output value responsive to the application of the force have the same sign (polarity), and thus, the sensor output value will be determined through same-polarity arithmetic operations.
FIG. 8 shows one example of a detection scheme, disclosed in the No. 2003-207405 publication, for detecting magnitude values and directions of six axial forces when an external force has been applied to the six-axis force sensor. To facilitate understanding, FIG. 8 shows deformation patterns 131 and deformation states in an exaggerated fashion. Any one or combination of six axial forces is applied, as an external force, to the operating part 121 of the force sensor chip 111. The operating part 121, to which an axial force has been applied, changes its position while being supported by the peripheral supporting part 122 and four connecting parts connecting the operating part 121 and peripheral supporting part 122. As a consequence, specific deformations, corresponding to the applied axial forces, are produced in the four connecting parts, and specific detection signals corresponding to the deformations are output via the strain resistance elements R11-R43.
More specifically, (1) of FIG. 8 shows axial forces Fx, Fz, My, Mz applied to the operating part 121, (2) of FIG. 8 shows deformation patterns of the force sensor chip 111 responsive to the applied axial forces Fx, Fz, My, Mz, and (3) of FIG. 8 shows mathematical expressions for determining strain characteristic of the applied axial forces Fx, Fz, My, Mz. As the deformation patterns of the force sensor chip 111, there are shown, in (3) of FIG. 8, a deformation pattern 131 in a planar shape and deformation pattern 132 in a vertical sectional shape. The mathematical expressions will also be referred to as “resistance-increase/decrease-value determining mathematical expressions”.
R11, R12, R13, R21, R22, R23, R31, R32, R33, R41, R42 and R43, used in the resistance-increase/decrease-value determining mathematical expressions shown in (3) of FIG. 8, represent resistance change amounts of corresponding ones of twelve strain resistance elements.
As shown in FIG. 8, the axial force Fx is applied as indicated by arrow 133, in response to which a detection signal, determined by the mathematical expression of “((R21−R23)+(R43−R41))/4”, is provided as a prominent output signal. The axial force Fz is applied as indicated by arrow 134, in response to which a detection signal, determined by the mathematical expression of “−(R12+R22+R32+R42)/4”, is provided as a prominent output signal. The axial force My is applied as indicated by arrow 135, in response to which a detection signal, determined by the mathematical expression of “(R12−R32)/2”, is provided as a prominent output signal. Further, the axial force Mz is applied as indicated by arrow 136, in response to which a detection signal, determined by the mathematical expression of “((R13−R11)+(R23−R21)+(R33−R31)+(R43−R41))/8”, is provided as a prominent output signal. By performing appropriate arithmetic operations (e.g., well-known matrix operations) on these detection signals, it is possible to identify the axial forces applied to the six-axis force sensor.
With the conventional force sensor chip disclosed in the No. 2003-207405 publication, only when the sensor output value of the axial force Fz is calculated or determined, the individual terms of the mathematical expression have the same sign, and thus, the sensor output value is determined through same-polarity arithmetic operations, as noted above.
Namely, in the case of application of the force Fz, the resistance elements in the force sensor, disclosed in the No. 2003-207405 publication, present element output changes of the same polarity, so that the arithmetic operations for determining a sensor output (i.e., sensor output determining arithmetic operations) comprise only additions. Consequently, when noise has been produced, the noise can not be canceled, which would become a cause that produces an unwanted drift noise component in an output signal of the sensor.